Z2 is very friendly for new track riders, as is Pacific Track Time - saying that because they seem to do a lot to acclimate new riders to the track. Between those two, I would recommend Z2 given their off session new rider ‘class’.
C@TT is the best for intermediate and faster...that isn’t to say they aren’t friendly to new riders, just that the others tend to cater to them more.
That said, all the providers have subtle culture differences...and until you’ve ridden enough to notice it, they’ll all likely feel the same until then.
Good luck, and have fun!